Method for detecting a vapor of an alkyl ester of phosphoric acid in ambient air

ABSTRACT

A method for detecting the presence of a vapor of an alkyl ester of phosphoric acid, particularly tributylphosphate, in ambient air. A sample of ambient air is heated to convert any alkyl ester of phosphoric acid to an alkene. The heated sample of air is then passed to a sample loop. Carrier air periodically carries a fixed volume of sample from the sample loop through a chromatographic column to a photoionization detector which detects the presence and amount of the alkene in the sample.

BACKGROUND OF THE INVENTION

This invention relates generally to the sensing of fluid vapors. Moreparticularly, this invention relates to the sensing of vapors from alkylesters of phosphoric acid which are present in hydraulic fluids.

Alkyl esters of phosphoric acid, and particularly, tributylphosphate,are common components of hydraulic fluids especially of the type used inaircrafts. As the vapors of such components may be toxic, there is aneed for a simple and reliable sensor to measure the presence of suchvapors in the ambient air, particularly at strategic locations within anaircraft so that any leakage of the vapor into ambient air can bedetected and appropriate measures be taken before any adverse effectupon personnel within the aircraft.

The sensor should be capable of measuring a concentration of such vaporsin the air as low as 400 parts per billion by volume. In addition, thesensor should be capable of forming very rapid analysis (less than 3minutes) and require little routine service and maintenance over a longperiod of time.

The sensor also must be selective to the particular alkyl ester ofphosphoric acid present in the hydraulic fluid. Accordingly, the sensingprocess must not be interfered with by any other trace chemicalimpurities that may be present in the ambient air, particularly that ofthe aircraft. Several of the potential chemicals which could presentinterference with the sensor include air (N₂ and O₂), water, sulfurdioxide, ozone, nitromethane, nitromethane, hydrogen chloride, andcarbon dioxide.

The monitoring of such vapors, especially tributylphosphate, presentsmany obstacles. Sampling of tributylphosphate vapor in the air undernormal conditions is virtually impossible since it absorbs on thesurface of the sampling system and requires considerable time tocompletely desorb. Moreover, tributylphosphate has a very high boilingpoint (26920 C.), at which temperature it decomposes.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a method and apparatus for the sensing of an alkyl ester ofphosphoric acid present in ambient air.

More particularly, it is an object of the present invention to provide amethod and apparatus for the sensing of tributyl phosphate vapor inambient air.

More specifically, it is an object of the present invention to provide amethod and apparatus for the sensing of an alkyl ester of phosphoricacid present as vapor in ambient air and which is selective to theparticular alkyl ester desired to be sensed and not interfered withother trace chemical impurities.

These and other objects of the present invention may be achieved inaccordance with the present invention through the provision of a methodfor detecting the presence of a vapor of an alkyl ester of phosphoricacid which comprises taking a sample of ambient air and treating thesample to convert the alkyl ester of phosphoric acid to an alkene. Thepresence of the alkene in the sample is then determined.

In accordance with the present invention, an apparatus for detecting thepresence of a vapor of an alkyl ester of phosphoric acid in ambient airmay comprise means for taking a sample of ambient air, and means forconverting the vapor of the alkyl ester of phosphoric acid into analkene. Means are provided for determining the presence of the alkene inthe treated sample.

The invention may be more understood by reference to the followingdetailed description and to the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

The figure is a schematic representation of an apparatus in accordancewith the present invention.

DETAILED DESCRIPTION

The figure is a schematic drawing showing an apparatus in accordancewith the present invention which is capable of detecting the presence ofa vapor of an alkyl ester phosphoric acid in ambient air. The apparatusis useful for many specific types of vapors, but is particularly usefulfor the sensing of tributylphosphate which is a component of hydraulicfluids used in aircrafts. Specific examples of hydraulic fluid whichcontain tributylphosphate and which is used in aircrafts include HyjetIV-A manufactured by Chevron and Skydrol LD-4 and Skydrol 500-B-4manufactured by Monsanto.

Referring to the figure, the apparatus includes a sensor housing 2indicated by the dotted lines. Carrier air under pressure from asuitable source is supplied through line 4 into the housing 2 through apressure regulating valve 6 which may be mounted external of the housing2 downstream of the valve 6. A pressure gauge 8 may be positioned in theline 4 downstream of the valve 6 for monitoring the pressure of thecarrier air. A portion of the air from line 4 is bled off through a line10 to an electrical heater 12, external of the sensor housing 2, toprovide heated air through line 14 into the interior of the sensorhousing 2 so that the sensor housing 2 provides a heated enclosure. Aproportional temperature controller 16, which may be external of thehousing 2 has a temperature sensor 18 mounted within the sensor housing2 to sense the temperature therein and provide feedback to theelectrical heater 12 to maintain the interior of the sensor housing at aconstant temperature. The temperature within the housing 2 should bemaintained at a temperature high enough above ambient so that changes inthe ambient temperature will not have a significant effect upon theinterior temperature. A temperature of from about 60° to 70° C. may beused as an example.

A portion of a carrier air is also bled off through a line 20 to anelectrically actuated solenoid valve 22. The solenoid 22 serves todirect the bled air to either side of the diaphram 24 of a sample valve26 positioned within the sensor housing 2 to cause the actuationthereof. The solenoid valve 22 is operated by electrical signals from aconsole unit 28 to direct air through either air line 30 or air line 32going to opposite sides of the diaphram 24 of the sample valve 26.

The sample valve 26 may be a 6-port valve with the piston element 34thereof moveable between a sample gathering position as shown in thefigure and a sampling position wherein the piston element is moved tothe right from its position shown in the figure.

The carrier air passes into the sensor housing 2 through line 4 andpasses through a carbon air filter 36. The air exits the filter througha line 38 attached to one port 40 of the sampling valve 26. In thesample-gathering position, the piston 34 of the sample valve 26 connectsthe incoming carrier air from line 38 with an outlet port 42 connectedto a line 44. The line 44 is connected to a chromatographic column 46which in turn is connected to a detector 48.

An aspirator 50, passing air therethrough, draws a suction through line52 attached to a port 54 of the sample valve 26.

The ambient air which is to be tested for the presence of the alkylester of phosphoric acid is drawn through a heated reactor 56. Thisheated reactor 56, which may be positioned immediately on the outside ofthe sensor housing 2, is used to convert the vapor of an alkyl ester ofphosphoric acid into its alkene component for sampling purposes. Theheated reactor 56 is a conventional type hot air heater. For example,the heater may be a 1000 Watt Air heater of the type manufactured byHotwatt, Inc. The heater relies upon the moisture vapor in the ambientair to provide the water necessary for the hydrolosis of the alkyl esterof phosphoric acid into its alkene component. By way of example, onealkyl ester of phosphoric acid for which this invention is particularlyadapted is tributylphosphate. As shown by the following chemicalequations, the hydrolysis of the tributylphosphate molecule producesbutene: ##STR1##

The moisture from the ambient air provides the water which is consumedin the first step chemical reactions. Thus, any tributylphosphate in theair entering the heated reactor 56 exits from the reactor 56 throughline 58 as butene which, under ordinary conditions, is a gas.

The temperature of the reactor and resident time of the ambient airshould be sufficient to ensure that the alkyl ester is broken down intoits alkene. Preferably, the temperature is maintained at at least 250°C. The residence time may be relatively short, in the order of less thana second. Line 58 containing the treated sample air extends from thereactor 56 into the sensor housing 2 and is connected to the port 60 ofthe sample valve 26.

A sample loop 62, preferably a coil of tubing, is provided which cancontain a predetermined volume of reacted ambient air for testing. Theinlet and outlet lines 64 and 66 to and from the sample loop 62 areconnected to the ports 68 and 70 respectively of the sample valve 26 asindicated in the figure. Thus, with the sample valve in itssample-gathering position, the aspirator 50 constantly draws sample airthrough the reactor 56, through line 58 into the sample valve 26, out ofthe sample valve 26 through line 64, through the sample loop 62 backinto the sample valve 26 through line 66 and out through line 52 to theaspirator 50.

At predetermined intervals, which may be preprogrammed by the controller28, when it is desired to test the sample of ambient air, a signal issent to the solenoid valve 22 to cause the valve 22 to switch to permitair to flow through line 32 to actuate the sample valve 26 and cause thepiston element 34 thereof to move to the right as viewed in the figure.In this position, the carrier air connected with line 64 going to thesample loop 62. The output line 66 from the sample loop 62 is connectedto the line 44 running to the chromatographic column 46. The line 58from the reactor 56 and the line 52 to the aspirator 50 are blocked bythe sample valve 26 during this interval.

During this testing period, the carrier air flows through the sampleloop 62 carrying a fixed volume of the sample of ambient air from thesample loop 62 through the chromatographic column 46 and detector 48.

The chromatographic column renders the detector 48 selective byspatially separating the butene, or other alkene, from the othercomponents of the ambient air as it passes through the column. Thecolumn may consist of a coil of packed tubing having a medium thereinwhich will cause the spatial separation of the alkene such as butene. Byway of example the column may comprise a coil of tubing packed withKel-F® oil, manufactured by Minnesota Mining and Manufacturing Company,deposited on diatomaceous earth. The tubing may be aluminum and vary inlength, although a 10 foot length has been used successfully.

The detector 48 is adapted to give an electronic output proportional tothe amount of butene present in the sample tested. A number of differenttypes of detectors may be suitable for the detection of butene or otheralkenes such as a flame ionization detector (FID) a photoionizationdetector (PID) and a discharge ionization detector (DID). The FIDdetector is generally not satisfactory for use in aircraft applicationsbecause of its flame. A DID detector also is generally inadequatebecause it possesses relatively little selectivity. Additionally, boththe DID and FID detectors require bottled gas such as belium andhydrogen, which also makes these impractical for use in aircraft.Accordingly, it is preferred to use a PID detector because of itsconsiderable selectivity. The detector may be any commercially availablephotoionization detector. A suitable detector is a 10.2 electron voltlamp detector manufactured by H.N.U. Systems Inc. The output signal fromthe detector 48 may be sent to the controller 28 or other device to beprocessed to provide an alarm when the detected concentration is over aminimum amount, an analog or digital readout, as well as to provide aprint out if desired.

The detector may be calibrated by passing an air sample through thesystem containing a known amount of tributylphosphate and observing thedetector signal output and then determining a correlation between theelectrical signal output of the detector and the amount oftributylphosphate in the sample air.

The sample valve 26, sample loop 62, the chromatographic column 46 anddetector 48 are all contained within the sensor housing 2 which ismaintained at a constant temperature through the electric heater 12 andproportional temperature controller 16. Thus, the temperature of thesample will remain constant and provide consistency between the vaporsamples.

During the sample-gathering period, when no detection is being carriedon, the carrier air line 4 and 38 is connected to the outlet line 44from the sample valve 26. This provides a supply of pressurized airthrough the chromatographic column 46 and detector 48 and then throughthe exhaust 72 from the detector 48 to purge the detection system.

While the invention has been described above with reference to aspecific embodiment thereof, it is apparent that many changes,modifications, and variations can be made without departing from theconcept disclosed herein. Accordingly, it is intended to embrace allsuch changes, modifications, and variations that fall within the spiritand broad scope of the appended claims. Any patent or patentapplications cited herein is incorporated by reference in its entirety.

What is claimed is:
 1. A method of detecting the presence of a vapor ofan alkyl ester of phosphoric acid in ambient air comprising the stepsof:a. taking a sample of ambient air, b. heating said sample to convertany alkyl ester of phosphoric acid to an alkene, and c. detecting thepresence of said alkene in said sample.
 2. The method of claim 1 whereinmoisture in the ambient air providing water during the heating of thesample to hydrolyze the alkyl ester of phosphoric acid to said alkene.3. The method of claim 1 wherein the detection of the presence of thealkene in said sample is performed by a photoionization detector.
 4. Themethod of claim 3 further comprising the step of passing said samplethrough a chromatographic column prior to detection of the presence ofthe alkene.
 5. The method of claim 1 wherein said alkyl ester ofphosphoric acid is tributylphosphate and said alkene is butene.
 6. Amethod of detecting the presence of a vapor of an alkyl ester ofphosphoric acid in ambient air comprising the steps of:a. taking asample of ambient sir, b. heating said sample to convert any alkyl esterof phosphoric acid in said sample into an alkene, c. passing said heatedsample of ambient air through a sample loop to provide a fixed volume ofsample, d. periodically passing a carrier fluid through said sample loopto carry the fixed volume sample in said loop to a detector, and e.determining the presence of the alkene in said fixed volume sample fromsaid sample loop when said fixed volume sample is passed to saiddetector.
 7. The method of claim 6 further comprising passing said fixedvolume sample from said sample loop through a chromatographic columnprior to passing it to said detector.
 8. The method of claim 7 furthercomprising passing said carrier fluid through said chromatographiccolumn and detector during the period when said fixed volume sample insaid sample loop is not passing to said chromatographic column anddetector.
 9. The method of claim 8 wherein said sample of ambient air isdrawn through said sample loop by an aspirator.
 10. The method of claim9 further comprising maintaining said sample of ambient air at aconstant temperature during its passage through said sample loop,chromatographic column and detector.